TCL - feeding STDIN to exec - tcl

Question

I'm trying to implement a retry mechanism when executing an external program using TCL. I'm having some issues when trying to feed STDIN to the external program. I'm now working with a simplified example trying to solve the issue. Take the following python script (simple.py):
x = raw_input()
y = raw_input()
print x + y
Inputs 2 strings from the output will be the concatenation result of the strings.
Now the following command works from the TCL interpreter:
% exec python stuff.py << 1\n2
12
However when I try to split it in separate commands, or add them to a string before doing this, it fails.
Fail 1:
% set cmd "python simple.py << 1\n2"
% exec $cmd
couldn't execute "python simple.py << 1
2": no such file or directory
Fail 2:
% set cmd1 "python simple.py"
% set cmd2 "1\n2"
% exec $cmd1 << $cmd2
couldn't execute "python simple.py": no such file or directory
Fail 3:
% set fullCommandString "exec python simple.py << 1\n2"
% eval $fullCommandString
Traceback (most recent call last):
File "simple.py", line 2, in <module>
y = raw_input()
EOFError: EOF when reading a line
The 3rd case seems that starts the script, but it interprets both lines of STDIN as one.
Any help is appreciated.

Answer 1

Tcl's commands do not reinterpret whitespace in their arguments by default. exec is one of these, and it follows the same rules. That means that you need to tell Tcl to interpret that list of words as a list of words as otherwise it is just a string. Fortunately, there's {*} for this; the expansion operator syntax interprets the rest of the word as a Tcl list, and uses the words out of that list at the point where you write it. It's very useful I find.
The simplest to fix is actually your second case:
% set cmd1 "python simple.py"
% set cmd2 "1\n2"
% exec {*}$cmd1 << $cmd2
You can fix the first and third by adding Tcl list quoting to ensure the 1\n2 is still interpreted as a single word (as otherwise newline is a perfectly reasonable list item separator).
% set cmd "python simple.py << {1\n2}"
% exec $cmd
% set fullCommandString "exec python simple.py << {1\n2}"
% eval $fullCommandString
The third can be written more economically though:
% set fullCommandString "exec python simple.py << {1\n2}"
% {*}$fullCommandString
As a rule of thumb, if you see eval in modern Tcl (note: not namespace eval or interp eval or uplevel) then it's usually an indication that some code could be made more efficient and to have fewer bugs by switching to using expansion carefully.
tl;dr: Put {*} before $cmd1 in your second example to get the idiomatic fix.